Transcription termination is a process whereby the elongation complex dissociates into RNA transcript, DNA template, and RNA polymerase (RNAP) in response to intrinsic DNA signals or specific factors. Both, intrinsic termination and factor-dependent termination are an integral part of the transcription cycle and play crucial role in regulating gene expression of bacteria and phages. Classical examples of such regulation include attenuation of transcription of biosynthetic operons and phage gamma-antitermination. The long-term objective of the proposed work is to provide a comprehensive structural and mechanistic description of intrinsic and Rho-dependent transcription termination processes in Escherichia coli and the mechanism of their positive and negative regulation by host factor NusA and phage XN protein, respectively. It is further proposed to examine the attenuation mechanism that controls nucleotide biosynthesis in Bacillus subtilis. Specific aims are: Complete studies on the mechanism of intrinsic termination and its control by N. The general mechanism of intrinsic termination and its regulation by N and NusA is now understood. However important details of the process remain unresolved. These include the positioning of N, NusA, and other Nus factors in the elongation complex, conformational changes in RNAP that accompany the termination process, and the active role of certain RNAP domains in termination. Experiments are proposed to address those questions. Analyze the mechanism of Rho-dependent transcription termination and its control by N. The rearrangement of protein-RNA and protein-DNA contacts in the elongation complex during termination by Rho will be monitored. Experiments are proposed to determine the effect of gamma N antitermination factor on the protein-nucleic acids interactions in the Rho-termination complex. Analyze the termination control mechanism of purine operons in Bacilli. Experiments are proposed to examine the role of newly discovered "riboswitches", i.e. natural regulatory RNA aptamers that directly sense small molecules, in regulation of purine operons in B.subtilis.